MIC2128 Datasheet, PDF (20/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

MIC2128
5.3 Setting the Soft-Start Time
The output Soft-Start time can be set by connecting a
capacitor from SS to A_GND from 2 ms to 100 ms as
shown in Figure 5-4.
MIC2128
COMPENSATION
gm
Comparator
VREF
0.6V
13 FB
ISS
1.3 µA
12 ^SS
C_SS
FIGURE 5-4:
Setting the Soft-Start Time.
The value of the capacitor can be calculated using
Equation 5-4.
EQUATION 5-4:
Where:
C_SS
I_SS
t_SS
V_REF
C_SS
=
I---_S--S----_{-----t--S---S}
V_REF
= Capacitor from SS pin to A_GND
= Internal Soft-Start current (1.3 µA typical)
= Output Soft-Start time
= 0.6V
5.4 MOSFET Selection
Important parameters for MOSFET selection are:
• Voltage rating
• On-resistance
• Total gate charge
The voltage rating for the high-side and low-side
MOSFETs are essentially equal to the power stage
input voltage V_IN. A safety factor of 30% should be
added to the V_IN(MAX) while selecting the voltage rating
of the MOSFETs to account for voltage spikes due to
circuit parasitic elements.
5.4.1
HIGH-SIDE MOSFET POWER
LOSSES
The total power loss in the high-side MOSFET
(P_HSFET) is the sum of the power losses because of
conduction (P_CONDUCTION), switching (P_SW), reverse
recovery charge of low-side MOSFET body diode
(P_Qrr) and MOSFET's output capacitance discharge as
calculated in the Equation 5-5.
EQUATION 5-5:
P_HSFET = P_{CONDUCTIONHS} + P_{SWHS} + P_Qrr + P_COSS
P_{CONDUCTIONHS} = I_{RMSHS}²  R_{DSON_HS}
P_{SWHS} = 0.5  V_IN  I_LOAD  t_R + t_F  f_SW
P_Qrr = V_IN  Q_rr  f_SW
P_COSS
=
1--
2

C_{OSSHS}
+
C_{OSSHS}

V_IN²

f_SW
Where:
R_{DS(ON_HS)} = On-resistance of the high-side MOSFET
V_IN
= Operating input voltage
I_LOAD
= Load current
f_SW
= Operating switching frequency
Q_rr
= Reverse recovery charge of low-side
MOSFET body diode or of external
diode across low-side MOSFET
C_OSS(HS)
= Effective high-side MOSFET output
capacitance
C_OSS(LS)
I_RMS(HS)
= Effective low-side MOSFET output
capacitance
= RMS current of the high-side MOSFET
which can be calculated using
Equation 5-6.
t_R, t_F
= The high-side MOSFET turn-on and
turn-off transition times which can be
approximated by Equation 5-8 and
Equation 5-9
EQUATION 5-6:
I_{RMSHS} = I_LOAD  D
I_LOAD is the load current and D is the operating duty
cycle, given by Equation 5-7.
EQUATION 5-7:
D = ^V----^O----^U----^T-
V_IN
DS20005620A-page 20
 2016 Microchip Technology Inc.